Transcript Xen and the Art of Virtualization

Xen and the Art of
Virtualization
Paul Barham, Boris Dragovic, Keir
Fraser, Steven Hand, Tim Harris, Alex
Ho, Rolf Neugebauer, Ian Pratt &
Andrew Warfield
Presented by Arthur Strutzenberg
What I plan to address
• Motivations for Virtualization
• How Xen works
• Xen vs. Bare Hardware vs. Disco/VMWare
– In this case it is sometimes easier to use
VMWare to compare against because (Xen
and VMware) they were both designed for the
x86 architecture
• The future of Xen
Virtualization & the Challenges
• Speed & Performance
• Security
– Resource Isolation
• Functionality
• Xen & its target
– The authors came up with the design goal of being
able to run 100 simultaneous virtual machine
implementations with Binary Compatibility
Breaking it Down
• Virtualization (today) can be broken down
into two main categories
– Full Virtualization
• This is the approach that Disco and VMWare uses
– Paravirtualization
• This is the approach that Xen uses
The Traditional Approach
• Traditional VMM (Virtual Machine Monitor)
exposes its “hardware” as being
functionally identical to the physical
hardware
– This approach can be difficult to implement
(especially with x86 systems)
– There are also situations where it is useful to
provide real AND virtual resources (for
example virtual and real timers)
• Under this model, the “guest” machine would not
have access to this information
Xen’s Approach
• Instead of making the virtual machine 100%
functionally identical to the bare hardware, Xen
makes use of Paravirtualization
• Paravirtualization is a process where the guest
operating system is modified to run in parallel
with other modified systems, and is designed to
execute on a virtual machine that has a ‘similar’
architecture to the underlying machine.
– Pros:
• Allows for improved performance
– Cons:
• The hosted operating system must undergo modification
before it can be hosted by the Xen Hypervisor (this can be a
bit of a challenge)
Xen’s Design Approach
Xen: Notation
• Guest Operating System
– The OS software that Xen hosts
• Domain
– The virtual machine within which a guest operating
system executes
• Guest OS’es and domains are analogous to a
program and a process
• Hypervisor
– This is the instance of Xen that handles all of the low
level functionaly
Under the hood
(how does Xen perform its magic?)
• The Xen paper discusses the following areas
– CPU
• Virtualization of the CPU
• CPU Scheduling
• Time & Timers
– Memory Management
• Virtual Address Translation
• Physical Memory
– Device I/O
• Network
• Disk
– Control Transfer
Xen and the CPU
• This undoubtedly where the most change
is required by the guest OS
• Xen challenges the assumption that the
OS is the most privileged entity
• Privileged instructions
– These are paravirtualized by requiring them to
be validated/executed within Xen
Xen and the CPU
• The x86 is less difficult than
most systems to virtualize
– This is due to the built in
security levels build within the
x86 (known as rings)
– Most systems have the OS
running on ring 0 (the most
privileged)
– Most user software runs on
ring 3
– Ring 1 & 2 generally are not
used
• Xen uses this fact to modify
the OS to execute on ring 1
Increasing Privilege Level
Ring 0
Ring 1
Ring 2
Ring 3
Xen, Scheduling, and Timers
• Xen currently uses an algorithm called the Borrowed
Virtual Time algorithm to schedule domains
• This is important to mitigate the problem of one domain
executing code that can adversely affect another domain.
• Xen also provides several different types of timers
– Real Time (time that always advances regardless of the executing
domain)
– Virtual Time (time that only advances within the context of the
domain)
– Wall Clock Time (time that takes in to account local offsets for time
zone and DST)
Control transfer & Eventing
• Exceptions and Eventing
– These include memory faults and software traps
– These are generally virtualized through Xen’s event
handler
– Typically the two most frequent exceptions that occur
(enough to effect performance)
• System Calls
• Page Faults
– These are two examples of a ‘fast’ handler (one in
which bypasses the hypervisor)
Paravirtualization of the MMU
Paravirtualization
Full Virtualization
Diagrams provided by a presentation from the Universität Karlsruhe
Xen and Virtual Memory
• When the guest OS requires a new page table, it
allocates it from its own memory reservoir
– After this it is registered with Xen
– The OS then gives up all direct write privleges to the
memory
– All subsequent updates must be validated by Xen
– Guest OS’s generally batch these update requests to
spread the cost of calling the hypervisor
• Segmentation is virtualized in a similar way
Xen and Virtual Memory
• Xen uses a design where
– Guest OS’s are responsible for allocation and
managing hardware pages
– Xen exists in a generally unused section at
the top of every address space. This is to
ensure that the Xen is never paged out
• This differs from the approach that Disco
takes where the Disco VMM keeps a
second level of indirection.
– Essentially VMM within VMM
Memory Management
• As discussed in an earlier class Memory
Management can be quite challenging
• Some key challenge points
– x86 does not have a software managed TLB
• Its TLB is not tagged, which means that the TLB
must be flushed on a context switch
Xen and Device I/O/ Management
• Data I/O is transferred to
and from domains via
Xen through the use of a
buffer descriptor ring
– This is a system that is
based around a pair of
producer consumer
pointers, one set used
within the guest OS, the
other within the Hypervisor
– This allows for the
decoupling of when data
arrives/is accessed and the
event notification
Control of the Hypervisor
• Domain0 is given greater
access to the hardware
(and hypervisor). It has a
guest OS running on top
of it as well, but also has
additional “supervisor”
software to manage
elements of the other
existing domains.
• This is different than
VMWare which has the
notion of a Host OS
acting underneath it.
Hypervisor Manager
Guest OS on Domain 0
Guest OS on Domain X
Domain 0
Domain 1
Hypervisor
Guest OS
Guest OS
VM Specific
Drivers
VM Specific
Drivers
VM
VM
VMWare
Host Operating System
VMWare
Drivers
Other
Drivers
Disk I/O (The Differences)
• Disco acts as the go between for Disk I/O
• Xen allows Domain0 to have direct access to the disk.
– Domain0 houses virtual block device (VBD) management software
– The VBD makes use of the ring mechanism
– Subsequent domains confine their disk access through the VBD
management software
– This allows Xen to maintain a tighter control over disk access, and to
allow “batching” of disk requests
• VMWare (from experience) allows for several options for Disk I/O.
– To allow the guest OS unfettered access to the raw device--basically as
a “pass through”
– Allow VMWare to create a “virtual disk” that is a binary file that is
contained within the file system of the host OS, and is controlled by the
VM Virtual Machine
• These are also different from running an OS on top of bare
hardware, where Disk I/O is managed by the OS
Building a new Domain on Xen
• Domain0 is a privileged domain
• New domain creation is delegated to
Domain0
– This offers the advantage of reducing the
complexity of the hypervisor
– Additionally building new domains that
originate from Domain0 allow for a better
debug environment
Networking
• Networking and Computers go
hand in hand today
• Because of this, Xen also
provides a “Virtual Firewall”
– Domain0 is responsible for
creating the firewall rules (can
we see a common theme
emerging?)
– Data is transmitted (and
received) using two buffer
rings (one for outgoing, the
other for incoming data)
– Incoming data packets are
analyzed by Xen against the
Virtual Firewall rules, and if
any are broken, the packet is
dropped
Internet
Xen’s Virtual Firewall
Guest0
Guest1
Guest2
Guest3
Other Hardware
• What was observable from the block diagram for
Xen was that you still have the notion of Xen
enabled hardware drivers
• This is similar to how VMWare operates.
– For instance if you have a sound card on your
machine, the hosting guest machine will detect that
you have a Sound Blaster enabled sound card.
– Another example (from VMWare) is the video driver
that can be installed on the guest OS to improve
video performance.
• This is another hidden challenge of virtualization
– Not only do you have to virtualize the memory and
CPU, but also any other devices that the guest OS
can access!
Xen and the Future
• This paper was presented October 2003. Since
then, the popularity of Xen has increased to
include support from vendors such as
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Sun Microsystems
Hewlett-Packard
Novell
Red Hat
Intel
Advanced Micro Devices
Voltaire
IBM
Xen and the Future
• To quote the news.com article (see my works cited list
for the complete article)
“The requirement for a modified operating system will
loosen with Intel’s coming Vanderpool Technology”
(Vanderpool is a hardware virtualization project)
• Additionally, AMD announced they are working on bring
Xen to their 64 bit platform
• Intel has experimental support on its Itanium chipset
• IBM is also working on a varient of the Hypervisor (a
“Secure Hypervisor”) that adds more protections against
attacks.
Works Cited
• I used several diagrams from the following paper
for this presentation
– http://i30www.ira.uka.de/teaching/coursedocuments/9
0/Xen.pdf
• This link provided some up to date info on Xen
– http://news.com.com/Xen+lures+bigname+endorsements/2100-7344_3-5581484.html
• Information about Vanderpool can be found here
– http://www.intel.com/technology/computing/vptech/